This invention relates to the field of AM stereo reception and, more particularly, to AM stereo receivers which include means for protecting reception from producing undesired side-to-side motion in the perceived stereo image (Platform Motion).
Platform Motion is an inherent characteristic of AM Stereo systems which utilize the phase relationship between the broadcast carrier and its sidebands to represent stereo difference (L-R) information. Such systems are called "phase separation" type systems in this specification. Phase separation AM stereo systems include AM/PM, AM/FM and quadrature modulation systems, including the pure quadrature amplitude modulation (QUAM) system, as proposed at one time by the Harris Corporation, and the so called compatible quadrature amplitude modulation system proposed by Motorola Inc. (the "Motorola system").
Platform Motion can result from several causes, one of which is co-channel interference (i.e., interference caused by the reception of two or more signals (a desired signal and one or more undesired signals) having almost identical carrier frequencies.
Co-channel interference produces a carrier beat between the desired and undesired signals. This introduces a resultant carrier wobble. Also, and more importantly, for medium and low level interference, the sidebands of the undesired or interfering signal swing in phase around the carrier of the stronger desired signal. In phase separation AM stereo systems the result is that the interference itself moves from side-to-side in the perceived stereo image, increasing the annoyance factor of the co-channel interference.
On the other hand, AM stereo systems that use the frequency difference between the upper and lower sidebands of the broadcast signal to represent stereo information [i.e., the Kahn/Hazeltine Independent Sideband (ISB) System where, for example, left stereo information is transmitted via the lower sideband and right stereo information via the upper sideband] do not suffer from Platform Motion.
U.S. Pat. No. 4,489,431, which issued to L. M. EcKlund, (the '431 Patent) discloses circuitry for use in AM stereo receivers for the Motorola system to avoid the annoying effects of Platform Motion. The '431 Patent's solution to the Platform Motion problem is to detect the presence of co-channel interference and switch the AM stereo receiver to monophonic operation when co-channel interference occurs. While this simple solution eliminates Platform Motion, it also obviously eliminates stereo reception at the same time, thereby reducing the stereo coverage of the desired AM station involved. Thus, those AM broadcast stations which use the Motorola AM stereo system and which suffer from Co-channel interference will have their stereo coverage significantly reduced.
The means for detecting co-channel interference disclosed in the '431 Patent may be used as part of AM stereo receivers which embody the present invention and, therefore, the specification and drawings of the '431 Patent are incorporated herein by reference.
It is, therefore, an object of the present invention to prevent co-channel interference from causing Platform Motion in AM stereo receivers for phase separation AM stereo systems, such as the Motorola system, while not switching such receivers to monophonic operation.
It is a further object of the invention to provide improved AM stereo receivers which avoid loss of stereo coverage for AM stations which use phase separation AM stereo systems and which are subject to co-channel interference.